1. Field of the Invention
The present invention relates to a two stage flash process for the effective removal of hydrocarbons in an ethylene oxide process carbon dioxide removal system whereby the amount of flash gas required to remove a certain amount of hydrocarbon in a carbonate removal system is substantially reduced as compared to conventional practices.
2. Description of the Prior Art
In processes such as those where ethylene oxide is formed by the oxidation of ethylene with molecular oxygen, carbon dioxide is also produced during the oxidation and it is necessary that the carbon dioxide be separated in order to prevent a build up of this product.
In commercial ethylene oxide processes the carbon dioxide removal is frequently accomplished by a hot carbonate system such as is described, for example, in U.S. Pat. Nos. 3,823,222 and 4,160,810. All or a portion of the reaction gas from the ethylene oxide reactor after ethylene oxide removal, which is rich in unreacted ethylene, is contacted with a solution of potassium carbonate and potassium bicarbonate whereby the carbonate reacts with the carbon dioxide in the reaction gas and is converted to the bicarbonate. The resulting solution rich in bicarbonate is regenerated by steam stripping to release the carbon dioxide and convert the bicarbonate back to carbonate for further use in the process.
The reaction gas contains a substantial concentration of unreacted ethylene, and in the process of absorbing the carbon dioxide a certain amount of this unreacted ethylene is dissolved in the absorbent solution. When the solution is directly steam stripped to release the carbon dioxide, the dissolved ethylene contained therein is released to the atmosphere with the stripped carbon dioxide. The release of ethylene is disadvantageous in that the permissible amount of ethylene released to the atmosphere is limited by local atmospheric pollution regulations. Also the ethylene has a value as a feed stock. Therefore, it is required and desirable to remove dissolved ethylene from the absorber solution and recover this ethylene.
In normal practice, a single stage flash is employed to flash the absorber solution with the flashed vapor passing to a recovery compressor in order to recover ethylene for recycle to the process.
The regulations on ethylene emissions often specify a fixed quantity of ethylene which can be released to the atmosphere independent of plant size. As the plant size increases it is, therefore, often the case that the percent removal required for the ethylene significantly increases with increased plant size, to maintain the absolute emission limit. As the percent removal increases, the amount of flash gas required per unit of ethylene increases and the amount of recycle carbon dioxide also increases.
For large plants the amount of recycle carbon dioxide in a single stage flash system can exceed 20% of the normal carbon dioxide removal requirement and the recovery compressor size can become disproportionately large compared to a smaller plant. In accordance with the present invention, the use of a two stage flash system can significantly reduce this problem, thus reducing the cost of the carbon dioxide removal system and also the cost and energy consumption of the recovery compressor.
In accordance with the present invention, the enriched absorber solution from hot carbonate scrubbing of ethylene oxide reaction gas, after the ethylene oxide removal, which enriched solution is high in bicarbonate and which contains dissolved ethylene, is passed from the absorber to a first flash zone wherein the bulk of the dissolved ethylene, oxygen, and other gases are removed from the liquid by flashing into the gas phase. The liquid from the first flash zone is sent to a second flash zone wherein conditions of the flashing are controlled to insure that such ethylene as remains in the liquid from the second flash after flashing carbon dioxide and ethylene will meet the appropriate emission regulations when the liquid is steam stripped to release carbon dioxide.